Clamp rgb channel values to {0...1} (i.e. 0x00 – 0xFF) when displaying as hex string

[Mark Pospesel]

Because we support extended RGB color space, individual channels can have values less than zero or greater than one. These do not format well for hexadecimal strings. For UIColor.rgbDisplayString we should clamp the channel values to {0...1} to avoid issues (and maybe append a ⚠️ emoji to the end of the string to indicate that its actual value is outside the bounds of the sRGB color space).